Semiconductor device and printed circuit board

ABSTRACT

For a multi-terminal semiconductor package, such as a BGA or a CSP, that handles high-speed differential signals, a high-speed signal is assigned to the innermost located electrode pad on an interposer substrate, and the electrode pad is connected to the outermost located ball pad on the interposer substrate. With this arrangement, the length of a plating stub can be considerably reduced, and the adverse affect on a signal waveform can be minimized. This arrangement is especially effective for differential signal lines.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device on which ismounted a semiconductor element, for transferring a high-speed signal,and a printed circuit board therefor, and particularly to the wiring ona resin substrate (an interposer) of a semiconductor package, such as aBGA or a CSP.

2. Description of the Related Art

Recent electronic apparatuses, such as personal computers, include anI/F such as a USB or an IEEE 1394. The I/F transmits a very high-speedsignal having a pulse width that corresponds, after being converted, toa frequency of several hundreds of megahertz. The speed of a signal hasbeen increased even more, and there is a demand for the transmission ofa signal that corresponds to a frequency of one gigahertz.

Further, multifunctional ICs and IC modules have been developed that arelike system LSI chips, and these ICs are mounted in multi-terminalpackages, such as BGAs or CSPs. That is, an IC having a high-speedsignal transmission I/F tends to be mounted in a multi-terminalsemiconductor package, such as a BGA or a CSP. Generally, in asemiconductor package, a semiconductor element is connected by wirebonding to electrode pads on a resin substrate (an interposer) whereonthe semiconductor element is mounted. These electrode pads are connectedto the interposer by signal lines extended radially on the interposer.The electrode pads are also connected through vias to ball pads that areprovided on the reverse face of the interposer to attach thesemiconductor package to a motherboard.

Gold plating is required for the electrode pads on the interposer. Inorder to perform the gold plating for the electrode pads, the electrodepads must be rendered conductive from the outer edge of the interposer.Therefore, in addition to wiring connected to the mounted semiconductorelement, other wiring is extended from the outer edge of the interposerto the individual electrode pads. Wiring extended from an individualelectrode pad to the outer edge of the interposer is called a “platingstub”. A plating stub has an open end at the outer edge of theinterposer, along the transmission line, and the length of the stub isabout 1 to 4 mm for a BGA package, of a peripheral type, with 1 mmpitches and four rows.

FIG. 8 is a partial, opened-up view of a semiconductor package employinga conventional BGA, for which, to simplify the explanation, the interioris shown. An interposer substrate 101 in FIG. 8 has a two-layerstructure; however, a multi-layer structure of three or more layers maybe used. A semiconductor element 110 mounted on the interposer substrate101 is connected to electrode terminals 103 a and 103 b on theinterposer substrate 101 by wires 102, and the electrode terminals 103 aand 103 b are connected to electrode pads 105 a and 105 b by signallines 104 a and 104 b. Plating stubs 106 a and 106 b are extended fromthe electrode pads 105 a and 105 b to the outer edge of the interposersubstrate 101. The electrode pads 105 a and 105 b are connected by vias107 a and 107 b, which are formed in the interposer substrate 101, toball pads 108 a and 108 b on the reverse surface of the interposersubstrate 101. The ball pads 108 a and 108 b are connected by solderballs 109 a and 109 b to a motherboard (not shown).

Generally, when a period during which a signal reciprocates along asignal line in the open state is longer than the rise time for thesignal, a reflected waveform occurs in the signal waveform and causeswaveform distortion. For a signal for which the waveform is trapezoidal,the rise time for the signal is generally equal to about 5% of thecycle. Therefore, for a conventionally employed signal having afrequency of 1 GHz, the cycle is about 1.0 nsec and the rise time, whichis 5% of the cycle, is 0.050 nsec. Through a calculation performed byemploying a signal transfer rate of 6 nsec/m for a common glass epoxysubstrate, the equivalent length obtained, for both directions is 8.30mm, and the wiring length obtained that corresponds to one direction is4.15 mm. That is, in the open state, a plating stub of about 1 to 4 mmin length does not greatly affect the quality of the waveform.

Furthermore, according to the description in Japanese Patent Laid-OpenPublication 2001-110927, ten times the high frequency component of asignal used for a semiconductor element is taken into account, and inthe open state, the length of a plating stub is designated as being lessthan ¼ of the upper limit wavelength in a frequency band (smaller than3.5 mm for a case wherein the frequency is 1 GHz).

However, the frequency of a signal used for the semiconductor elementhas been repeatedly increased, and a signal having a frequency evengreater than 2 GHz is now employed. For a signal having a frequency of 2GHz, the cycle is 0.5 nsec and the rise time, which is 5% of the cycle,is 0.025 nsec. Through calculations performed using the signal transferrate of 6 nsec/m, the equivalent length in both directions is 4.15 mm,and the wiring length corresponding to one direction is 2.08 mm. Thatis, in the open state, a plating stub of about 2 mm or longer wouldgreatly affect the waveform of a signal to be transmitted.

As is apparent from the conventional semiconductor package shown in FIG.8, the plating stub 106 a extending from the outer electrode pad 105 ais short, and the plating stub 106 extending from the inner electrodepad 105 b is long. Therefore, in order for the length of the platingstub, for the wiring along which a high-frequency signal is transmitted,to be less than ¼ of the upper limit wavelength for the frequency band,as described in Japanese Patent Laid-Open Publication 2001-110927, theouter electrode pad 105 a must be employed as the electrode pad to beconnected to the wiring described above.

When there is such a restriction, much of the flexibility of the designof the interposer, and of the motherboard, is lost, and not only aredesign costs increased, but also, the preparation of a viable designtherefor would be impossible.

Further, since recently a differential transmission method is frequentlyadopted for high-speed signals, differential pair of signal lines forwhich impedance matching is required must be provided on the interposer.In order to acquire impedance matching for the differential pair ofsignal lines, a predetermined clearance must be maintained between twosignal lines of a differential pair of signal lines. However, it is verydifficult, while maintaining this clearance, for the differential pairof signal lines to be passed through a number of electrode pads andconnected to the electrode pads nearest the outer edge of the interposersubstrate. Therefore, in order to match the differential impedances ofthe differential pair of signal lines, these wires must be connected tothe innermost electrode pads on the interposer, so that the lengths ofthe plating stubs are increased, and the distortion of waveforms forsignals to be transmitted can not be avoided.

SUMMARY OF THE INVENTION

The present invention provides a semiconductor device that reduceshigh-speed signal waveform distortion, which is due to plating stubsformed on the interposer of a semiconductor package, such as a BGA or aCSP, and a printed circuit board on which this semiconductor device ismounted.

According to this invention, a semiconductor device has a plurality ofpads, wherein a first electrode pad is connected to a semiconductorelement by a first signal line and is also connected, by a via, to a padprovided on a reverse face of an interposer substrate. A secondelectrode pad is connected to the first electrode pad by a second signalline and is located nearer an outer edge of the interposer substratethan the first electrode pad that is connected to the pad on the reverseface of the interposer substrate. Also, a third signal line extends fromthe second electrode pad to the outer edge of the interposer substrate.These electrode pads and the third signal line are formed on the obversesurface of the interposer substrate on which the semiconductor elementis mounted.

Further, according to the semiconductor device of the invention, thefirst electrode pad is the innermost such pad on the interposersubstrate, and the second electrode pad is the outermost.

According to the semiconductor device of the invention, the first signalline, the second signal line, and the third signal line are adifferential pair of signal lines, for each of which two signal linesare arranged in parallel at a predetermined interval.

According to the semiconductor device of the invention, the length ofthe third signal line is 2 mm or less.

Furthermore, according to the invention, a printed circuit board isprovided on which the semiconductor device is mounted, wherein the firstelectrode pad is connected to a fourth signal line, on a motherboard,through a via and the pad arranged on the reverse face of the interposersubstrate. The second electrode pad is connected to a fifth signal line,on the motherboard, through a via and a pad arranged on the reverse faceof the interposer substrate and is also terminated by a terminatingresistor.

In addition, according to the invention, a printed circuit board isprovided on which the semiconductor device is mounted, wherein the firstelectrode pad and the second electrode pad are respectively connected,by a fourth signal line on a motherboard, through the vias and the padsarranged on the reverse face of the interposer substrate, and the secondelectrode pad is further connected to a fifth signal line on themotherboard.

Further features and advantages of the present invention will becomeapparent from the following description of exemplary embodiments (withreference to the attached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing a semiconductor device accordingto a first embodiment of the present invention.

FIG. 2 is a schematic diagram showing a line model for the semiconductordevice according to the first embodiment.

FIG. 3 is a schematic diagram showing a line model for a conventionalsemiconductor device.

FIG. 4 is a schematic perspective view of a semiconductor deviceaccording to a second embodiment of the invention.

FIG. 5 is a schematic top view of the semiconductor device according tothe second embodiment.

FIG. 6 is a schematic top view of a semiconductor device according to athird embodiment of the present invention.

FIG. 7 is a schematic perspective view of a semiconductor deviceaccording to a fourth embodiment of the present invention.

FIG. 8 is a schematic perspective view of a conventional semiconductordevice.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention will now be described.

First Embodiment

FIG. 1 is a schematic diagram showing a semiconductor package, accordingto a first embodiment of the present invention, that uses a BGA. Asemiconductor element 10, which is mounted on an interposer substrate 1,is connected to an electrode terminal 3 on the interposer substrate 1 bya bonding wire 2. The electrode terminal 3 is connected, by a signalline 4, to an electrode pad 5, the innermost located one, on the obversesurface of the interposer substrate 1. The electrode pad 5 is further.connected to an electrode pad 15, the outermost located one on theobverse surface of the interposer substrate 1, through a signal line 14that extends across the obverse surface of the interposer substrate 1. Aplating stub 6 is so formed that it extends from the electrode pad 15 tothe outer edge of the interposer substrate 1. Since the plating stub 6extends outward from the outermost located electrode pad 15 on theinterposer substrate 1, this is a very short line.

The electrode pad 5 is connected to a ball pad 8 on the reverse surfaceof the interposer substrate 1 by a via 7 formed in the interposersubstrate 1. The ball pad 8 is attached to a motherboard by a solderball 9, and is connected across a signal line 11 to a receiver element13 on the motherboard. The electrode pad 15 is connected to a ball pad18 on the reverse surface of the interposer substrate 1 by a via 17 thatis formed in the interposer substrate 1. The ball pad 18 is attached tothe motherboard by a solder ball 19, and is connected across a signalline 12 to a terminating resistor 20 on the motherboard. In order toacquire a match between the signal line 14 on the interposer substrate 1and the signal line 12 on the motherboard, the same line impedance mustbe set for the signal line 14 and the signal line 12, and a signal canbe terminated when the resistance of the terminating resistor 20 is setequal to the line impedance.

The semiconductor element 10 and the electrode terminal 3 may beconnected by flip chip bonding. Furthermore, in this embodiment, thesemiconductor element 10 arranged on the interposer substrate 1 isemployed as a driver and the receiver element 12 is arranged on themotherboard; however, the present invention is also effective when thearrangement is reversed.

Further, in this embodiment, on the obverse surface of the interposersubstrate 1, the innermost located electrode pad 5 and the outermostlocated electrode pad 15 are employed. However, the present invention isnot limited to this, and so long as a desired length or less can beprovided for the plating stub 6, the electrode pad 15 need only belocated nearer the outer edge of the interposer substrate 1 than is theelectrode pad 5.

A schematic line model for the structure in FIG. 1 is shown in FIG. 2.In FIG. 2, components corresponding to those in FIG. 1 are denoted bythe same reference numerals. For comparison, a schematic line model fora conventional structure in FIG. 8 is shown in FIG. 3. As is apparentfrom FIGS. 2 and 3, the plating stub 6 in FIG. 2 is considerably shorterthan a plating stub 106 in FIG. 3.

With this arrangement, the lengths of all plating stubs for transmittinghigh-speed signals, which has become an issue, can be 2 mm or shorter,and the adverse affect on signal waveforms can be minimized.

Second Embodiment

FIG. 4 is a schematic diagram showing a semiconductor package, accordingto a second embodiment of the present invention, that uses a BGA. Asemiconductor element 30 is mounted on an interposer substrate 21, andis connected to electrode terminals 23 a and 23 b on the interposersubstrate 21 by wires 22 a and 22 b. The electrode terminals 23 a and 23b are connected by signal lines 24 a and 24 b to electrode pads 25 a and25 b, the innermost located ones on the obverse surface of theinterposer substrate 21. Since the signal lines 24 a and 24 b aredifferential pair signal lines, the electrode pads 25 a and 25 b arepositioned adjacent to each other, and the signal lines 24 a and 24 bare extended outward while a predetermined interval is maintainedbetween them.

The electrode pads 25 a and 25 b are connected to electrode pads 35 aand 35 b, the outermost located ones on the obverse surface of theinterposer substrate 21, by signal lines 34 a and 34 b extending acrossthe obverse surface of the interposer substrate 21. Plating stubs 26 aand 26 b are formed so as to extend from the electrode pads 35 a and 35b to the outer edge of the interposer substrate 21. Since the platingstubs 26 a and 26 b extend outward from the outermost located electrodepads 35 a and 35 b on the interposer substrate 21, these are very shortlines. The electrode pads 25 a and 25 b are connected to ball pads 28 aand 28 b, arranged on the reverse surface of the interposer substrate21, by vias 27 a and 27 b formed in the interposer substrate 21. Theball pads 28 a and 28 b are attached to a motherboard via solder balls29 a and 29 b, and are connected to a receiver element 33 on themotherboard by lines 31 a and 31 b. The electrode pads 35 a and 35 b areconnected to ball pads 38 a and 38 b, arranged on the reverse surface ofthe interposer substrate 21, by vias 37 a and 37 b that are formed inthe interposer substrate 21. The ball pads 38 a and 38 b are attached tothe motherboard via solder balls 39 a and 39 b, and are connected bylines 32 a and 32 b to terminating resistors 40 a and 40 b on themotherboard. In order to acquire a match between the signal lines 34 aand 34 b on the interposer substrate 21 and the signal lines 32 a and 32b on the motherboard, the same line impedance must merely be set for thesignal lines 34 a and 34 b and the signal lines 32 a and 32 b, and asignal can be terminated when the resistance of the terminating resistor40 is set equal to the line impedance.

FIG. 5 is a schematic top view of the semiconductor package in FIG. 4,and components corresponding to those in FIG. 4 are denoted by using thesame reference numerals. As is apparent from FIG. 5, the signal lines 24a and 24 b are provided while a predetermined interval is maintained.

With this arrangement, even when the differential lines, for which thedifferential impedance matching is required, are employed, the length ofall the plating stubs for transmitting high-speed signals, which is anissue, can be 2 mm or less, and the adverse affect on the signalwaveforms can be minimized.

The present invention can also be applied for a semiconductor packagewherein more rows of electrode pads are arranged on an interposersubstrate, such as the one shown in FIG. 5 wherein three rows ofelectrode pads are provided for an interposer substrate. In the secondembodiment, the innermost located electrode pads 25 a and 25 b on theobverse surface of the interposer substrate 21 and the outermost locatedelectrode pads 35 a and 35 b are employed. However, the presentinvention is not limited to this, and so long as the length of theplating stubs is a desired length or less, the electrode pads 35 a and35 b need only be located nearer the outer edge of the interposersubstrate 21 than are the electrode pads 25 a and 25 b.

Third Embodiment

FIG. 6 is a schematic diagram, according to a third embodiment of thepresent invention, showing a semiconductor package that uses a BGA. InFIG. 6, as in the second embodiment, a differential pair of signal linesis employed for a semiconductor package that uses a BGA. In the thirdembodiment, four rows of electrode pads are arranged on an interposersubstrate 41.

In FIG. 6, an electrode terminal 43 a is connected by a signal line 44 ato an electrode pad 25 a on the second row from the innermost positionon the obverse surface of the interposer substrate 41. An electrodeterminal 43 b is connected by a signal line 44 b to an electrode pad 45b, the innermost located pad on the obverse surface of the interposersubstrate 41. Since the signal lines 44 a and 44 b are a differentialpair of signal lines, these lines 44 a and 44 b are extended outwardwhile a predetermined interval is maintained between them.

An electrode pad 45 a is connected by a signal line 54 a, provided onthe obverse surface of the interposer substrate 41, to an electrode pad55 a, the outermost one on the obverse surface of the interposersubstrate 41. An electrode pad 45 b is connected by a signal line 54 b,provided on the obverse surface of the interposer substrate 41, to anelectrode pad 55 b in the second row from the outermost location on theobverse surface of the interposer substrate 41. A plating stub 46 aextends from the electrode pad 55 a to the outer edge of the interposersubstrate 41, and a plating stub 46 b extends from the electrode pad 55b to the outer edge of the interposer substrate 41.

With this arrangement, the flexibility can be obtained for thepositioning of the electrode pads 45 a, 45 b, 55 a and 55 b, the linedensity on the interposer substrate can be reduced, and the design andmanufacture can be simplified.

Fourth Embodiment

FIG. 7 is a schematic diagram showing a semiconductor package, accordingto a fourth embodiment of the present invention, that uses a BGA. InFIG. 7, the terminating resistor 20, on the motherboard in FIG. 1 forthe first embodiment, is removed. The same reference numerals as areused in FIG. 1 are employed to denote corresponding components in FIG.7, and no further explanation for them will be given.

In FIG. 7, solder balls 9 and 19 are connected by a signal line 72, andsolder ball 19 is also connected to a receiver element 13 by a signalline 73.

In order to remove the terminating resistor 20 on the motherboard in thefirst embodiment, the signal lines extending from the ball pad 9 and theball pad 19 are also connected on the motherboard. At this time, whenthe characteristic impedance of a signal line 14 laid on the interposersubstrate 61 is equal to the characteristic impedance of the signal line72 laid on the motherboard, and the lengths of the two lines are equal,these lines can be regarded as a single line having a desiredcharacteristic impedance and are used for signal transmissions.Therefore, a signal transmitted by the ball pad 19 need not beterminated on the motherboard, the arrangement of a terminating resistoris not required, and the adverse affect on a reflected waveform, due tothe plating stub, can be reduced.

According to the present invention, for a multi-terminal semiconductorpackage such as a BGA or a CSP, a high-speed signal is assigned to theinnermost located electrode pad on an interposer substrate, and theelectrode pad is connected to the outermost located ball pad on theinterposer substrate. With this arrangement, the length of the platingstub can be considerably reduced, and the adverse affect on a signalwaveform can be minimized.

Further, according to the invention, the line impedance of a signalline, which extends from the outermost located electrode pad through thevia, the ball pad and the solder ball and is connected to themotherboard, is terminated by the terminating resistor for whichimpedance matching is acquired. Therefore, the affect on a waveform dueto the plating stub can be even more reduced.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed embodiments. On the contrary, the invention isintended to cover various modifications and equivalent arrangementsincluded within the spirit and scope of the appended claims. The scopeof the following claims is to be accorded the broadest interpretation soas to encompass all such modifications and equivalent structures andfunctions.

This application claims priority from Japanese Patent Application No.2004-121434 filed Apr. 16, 2004, which is hereby incorporated byreference herein.

1. A semiconductor device, which has a plurality of packaging pads, comprising: a semiconductor element; and an interposer substrate on which the semiconductor element is mounted, wherein the interposer substrate includes: a first electrode pad, which is connected to the semiconductor element by a first signal line, and which is also connected, by a via, to a pad provided on a reverse face of the interposer substrate, a second electrode pad, which is connected to the first electrode pad by a second signal line, and which is located nearer to an outer edge of the interposer substrate than the first electrode pad that is connected, by the via, to the pad on the reverse face of the interposer substrate, and a third signal line, which extends from the second electrode pad to the outer edge of the interposer substrate, wherein the first and second electrode pads and the third signal line are formed on an obverse surface of the interposer substrate on which the semiconductor element is mounted.
 2. A semiconductor device according to claim 1, wherein the first electrode pad is located on the interposer substrate inwardly of the second electrode pad.
 3. A semiconductor device according to claim 1, wherein each of the first signal line, the second signal line, and the third signal line includes a differential pair of signal lines, and wherein two signal lines of the differential pairs of signal lines are arranged in parallel at a predetermined interval.
 4. A semiconductor device according to claim 1, wherein a length of the third signal line is 2 mm or less. 5-8. (canceled)
 9. A semiconductor circuit assembly, comprising: a motherboard; and a semiconductor device mounted on the motherboard, wherein the semiconductor device includes a semiconductor element and an interposer substrate on which the semiconductor substrate is mounted, and wherein a first electrode pad is connected to the semiconductor element by a first signal line, a second electrode pad is connected, by a via, to the first electrode pad by a second signal line and is located nearer to an outer edge of the interposer substrate than the first electrode pad that is connected, by the via, to the pad on the reverse face of the interposer substrate, and a third signal line extends from the second electrode pad to the outer edge of the interposer substrate, wherein the first and second electrode pads and the third signal line are formed on an obverse surface of the interposer substrate, and wherein the first and the second electrode pads are connected to a fourth signal line, on the motherboard, respectively through first and second vias to the first and second pads arranged on the reverse face of the interposer substrate, and the second electrode pad is also connected to a fifth signal line, on the motherboard, through the second via.
 10. A semiconductor circuit assembly according to claim 9, wherein the first electrode pad is an innermost located pad provided for the interposer substrate, and the second electrode pad is an outermost located pad on the interposer substrate.
 11. A semiconductor circuit assembly according to claim 9, wherein a length of the third signal line is 2 mm or less. 